This invention relates to a method of encoding a first signal, for example, a speech parameter such as the pitch, as a function of time, to form a second signal, which second comprises a sequence of successive information blocks, an information block containing time information corresponding to a specific time instant, and containing amplitude information associated with said specific time instant, which amplitude information has been derived from the first signal. The invention also relates to a device for carrying out the method.
It is known to encode a signal, for example, a speech parameter such as the pitch in a speech signal, by determining the extrema in the signal, i.e. the relative and absolute minima and maxima in the signal. Subsequently, the signal is encoded into a sequence of information blocks, each information block indicating the instant at which an extremum occurs in the signal and the associated value of the extremum at this instant.
The encoded signal, which is constituted by the sequence of information blocks, can subsequently be transmitted via a transmission medium at a substantially lower bit rate than if the original signal were transmitted via the transmission medium. This is because the encoding provides a significant data reduction, enabling the signal to be transmitted via a transmission medium having a limited bandwidth. After reception of the encoded signal the original signal can be reconstructed by interpolation. The simplest interpolation is that in which the signal at instants situated between the instants of two successive information blocks is obtained by means of a straight line interconnecting two points defined by the information in two successive information blocks.
Another possibility is to reconstruct the original signal in that the information in the information blocks which relates to the magnitude of the first signal is approximated by a higher-order curve.
The reconstructed signal, for example, the pitch as a function of time, can subsequently be used to resynthesize a speech signal, for example by means of a speech chip. An example of such a chip is the N. V. Philips speech chip PCF 8200, as described in the Elcoma publication No. 217, entitled "Speech Synthesis: the complete approach with the PCF 8200".
The known method has the disadvantage that encoding is not always accurate enough and sometimes fails completely, for example, with respect to the pitch. From the publication "An efficient encoding method for electrocardiography using spline functions" by H. Imai et al., Systems and Computers in Japan, 1985, No. 3, May-June, pp. 85-94, a method is known which enables the signal to be encoded more accurately. In accordance with this method a third signal is derived from the first signal, which third signal is a measure of the curvature of the first signal as a function of time, extrema in said third signal are determined, and the first signal is encoded in the form of a sequence of information blocks, of which an information block contains time information corresponding to the instant at which an extremum occurs in the third signal. Determining the extrema in the curvature of the signal and encoding a signal on the basis thereof in this way yields a better approximation to the first signal.
An example of this is the encoding of a first signal which decreases continuously between a (relative) maximum and a (relative) minimum in conformity with two lines having different slopes and joining one another in a break-point situated between the instants at which the (relative) maximum and the (relative) minimum occur. The first-mentioned encoding method would yield two information blocks corresponding to the instants at which the (relative) maximum and the (relative) minimum occur and, for example, the associated values for the maximum and minimum. After decoding this would yield a reconstructed signal which varies between the maximum and the minimum in accordance with a straight line. The reconstructed signal no longer exhibits the break-point.
The second mentioned known encoding method allows for this break-point. The break-point yields a maximum or a minimum in the curve representing the curvature, so that also for this break-point an information block is generated. This information block indicates the instant at which the break-point occurs and, for example, the value of the original signal at this instant. When the information blocks are decoded this break-point again occurs in the reconstructed signal.